Ball-end mills have conventionally been used to three-dimensionally cut flat surfaces and curved surfaces of works such as molding dies, etc. To carry out the three-dimensional finishing of works with good surface roughness by ball-end mills, cutting edges should be free from chipping and cracking, with suppressed vibration for improved chip removal. To this end, it is important to adjust rake angles of arcuate cutting edges of ball-end mills. Accordingly, various proposals have been made so far, on the rake angles of arcuate cutting edges.
JP 10-80815 A discloses a ball-end mill suitable for three-dimensional cutting of curved surfaces of molding dies, etc., which has an rake angle of −2° to −20° in peripheral cutting edges for strengthening, and an rake angle of 0° to +10° near an axial center for better chip removal. Specifically, it describes an example in which the rake angle is +3° near a tip end, and −10° near peripheral cutting edges. However, because the rake angle of the cutting edge at the most projecting position is negative, this ball-end mill has poor capability of finishing works with high accuracy.
JP 2008-110437 A proposes a CBN ball-end mill comprising ball-end cutting edges and peripheral cutting edges, a normal rake angle of each ball-end cutting edge being −5° to −15° at R10°, −5° to +3° in a range of R50° to R70° and having a peak, and −10° to 0° at R90°, thereby suppressing chipping in the entire ball-end cutting edges for a long life. A specific example of the normal rake angles of the ball-end cutting edges is −10° at R10°, peak with 0° at R60°, and −5° at R90°, with gradual change in a plus direction in a range from R10° to R60°, and gradual change in a minus direction in a range from R60° to R90°. However, because this ball-end mill has a peak in a normal rake angle in a range from R50° to R70°, with a larger negative normal rake angle at R10° than at R90°, it suffers poor capability of finishing works with high accuracy.
JP 8-118133 A proposes a ball-end mill for cutting relatively soft works such as wood, non-ferrous metals, etc. smoothly with high accuracy, which comprises curved cutting edges, a rake angle of each cutting edge being 10-30° in a tip end portion with end cutting edges, and 20-40° in a peripheral portion, with an intermediate rake angle continuously changing from the rake angle of the end cutting edge to that in the peripheral portion in a corner portion. An example of the rake angles is 10° in the tip end portion and 20° in the peripheral portion, and another example is 20° in the tip end portion and 30° in the peripheral portion. Because the rake angle of each cutting edge is larger in the peripheral portion than in the tip end portion, and intermediate between those of the tip end portion and the peripheral portion in the corner portion, this ball-end mill cannot be used to finish high-hardness works (Rockwell hardness: 40 HRC or more) such as molding dies, etc.
JP 2004-181563 A proposes a ball-end mill comprising ball-end cutting edges whose center cutting edges are formed by rake faces; a clearance angle of each ball-end cutting edge being smaller than that of each center cutting edge; and each ball-end cutting edge having a normal rake angle gradually increasing to the positive side from a center toward a periphery, thereby having improved strength and chip removal. For example, a center cutting edge of each ball-end cutting edge has a normal rake angle of −45° at center, and gradually and positively increasing to −10° toward the periphery. However, because each center cutting edge has a largely negative normal rake angle, this ball-end mill has poor capability of finishing works with high accuracy.
Japanese Utility Model 62-12503 Y2 proposes a ball-end mill comprising ball-end cutting edges looked an S shape when viewed from the tip end side; a normal rake angle of each ball-end cutting edge being negative at a rotation axis center, and positive with gradual increase on the periphery side. However, because the rake angle of each ball-end cutting edge gradually increases on the positive side from the rotation axis center toward the periphery side for improved chip removal and cutting edge strength, this ball-end mill has poor capability of finishing works with high accuracy.
JP 2004-291096 A proposes a throwaway chip having twisted arcuate cutting edges; the thickness of a chip body at a position perpendicular to its rotation axis being in a range of 0.5 D to 0.9 D, wherein D is the thickness (mm) of a flat plate portion of the chip body; and the radial angle being 40-70° at a point most projecting in a rotation direction. However, JP 2004-291096 A never discloses the change of a rake angle depending on the radial angle of the throwaway chip. In addition, this throwaway chip does not have twisted peripheral cutting edges connected to rear ends of arcuate cutting edges. Accordingly, it is not suitable to three-dimensionally finish works having vertical walls with good surface roughness.